The present invention relates generally to cycloidal rotors, and more specifically to a new ring cam and ring cam assembly for dynamically controlling the pitch of cyclorotor blades at diverse pitching schedules, and for use with any apparatus where dynamic variation of cam controlled movement is needed.
A cyclogyro is an aircraft capable of vertical takeoff and landing using a horizontal axis propulsion concept known as a cycloidal rotor, or cyclorotor. FIG. 1 shows a cyclogyro and cyclorotor proposed and patented (U.S. Pat. No. 2,123,916) in 1933 by Rohrbach, but never built.
Cyclorotors are characterized by the rotation of blades about an axis where the span of the blades is parallel to the axis of revolution of the cyclorotor and perpendicular to the direction of flight. Aerodynamic forces are generated by cyclically pitching the blades forward and back as they move around the rotational axis. The manner in which the blades pitch during a rotation is known as a “pitching schedule.” For example, in a hovering flight condition, a positive pitch on the top portion of the cycle and a negative pitch on the bottom portion produce an upward force. By altering the pitching schedule, a cyclorotor can produce thrust in any direction perpendicular to its rotational axis.
A cyclorotor's ability to achieve higher aerodynamic efficiency and operate more quietly than traditional rotors, combined with an ability to nearly instantaneously manipulate both magnitude and direction of its thrust vector, as well as potentially optimize blade kinematics for every flight condition, makes it an attractive propulsion system for micro air vehicles (MAVs). Additionally, newer lightweight and high strength composite materials, combined with ongoing research and development, have begun to make cyclorotors practical for larger aircraft such as airships and vertical takeoff and landing (VTOL) aircraft. Many technical barriers, however, remain before cyclorotors can be practically implemented on any flying vehicle.
The problem is similar to that faced by the Wright brothers in solving the problem of controllable flight. The Wright brothers did not invent a winged airplane. They invented an apparatus and method for controlled flight for a winged airplane. Their solution was wing warping, later interpreted to include modern ailerons as functional equivalents.
A successful cyclorotor, for use in a successful cyclogyro, requires a solution similar to that of the Wright brothers. Central to that solution is development of a mechanism to produce required dynamic blade pitching kinematics.
The term dynamic is used to indicate varying cyclic control, usually controlled variation according to need, as opposed to a fixed cycle. For purposes of a cyclorotor, while the pitch of each cyclorotor blade is varied during a particular flight profile, that variation pattern typically is fixed. Dynamic control means that the variation pattern of blade pitch can be changed as needed for different flight profiles.
While the meanings of terms and terminology used for cyclogyros and cyclorotors is still evolving, and are often different from the meanings given for identical terms and terminology used for helicopters, a brief description of the meanings for cyclic pitch control and collective pitch control as applied to helicopter rotors and blades will make it easier to understand the meaning of the same terms as applied to cyclorotors.
The simplest helicopters use fixed pitch rotor blades. The amount of upward thrust is controlled entirely by the rotational speed of the rotor. However, by varying the pitch of each blade as it rotates, so that, for example, the pitch is reduced as the blades rotate toward the front of the helicopter, and is increased as the blades rotate toward the rear of the helicopter, the helicopter will tilt, or pitch (yet another meaning for the term “pitch”), forward, creating forward thrust. Similarly, if the pitch of the blades is reduced as they rotate toward the right side of the helicopter and the pitch is increased as they rotate toward the left side of the helicopter, the helicopter will tilt, or roll, toward the right, creating a thrust to the right. This type of pitch control, which varies in a fixed manner as the helicopter rotor blades cycle through a rotation, is called cyclic pitch control.
As just stated, for fixed pitch helicopter rotor blades, the amount of thrust is controlled by the rotational speed of the rotor. Even for cyclically controlled pitch helicopter rotors, the total thrust, while shared between up, forward or reverse and left or right, is mostly determined by the rotational speed. Collective pitch control changes the pitch of all the rotor blades at the same time in the same direction, thereby changing the total thrust, without having to change rotational speed, even while the blades may be undergoing cyclic pitch changes for pitch and roll.
The ability to dynamically control the pitch of helicopter rotor blades both cyclically and collectively was critical to the commercial success of helicopters.
While the meaning of cyclic pitch control and collective pitch control as used for cyclorotors may not yet be fixed, simple observation reveals that cyclic pitch control for a cyclorotor primarily changes the direction of thrust, and not pitch and roll as in a helicopter. Collective pitch control changes the magnitude of thrust. Changing the magnitude and direction of thrust on separate rotors can maneuver the vehicle.
The first free flying cyclorotor aircraft, or cyclogyro, was flown by a team at the University of Maryland in April 2011. Its four bar linkage pitch control can achieve only a cyclic pitch control, affecting in a cyclorotor the resulting direction of thrust, and not collective pitch control. Most cyclorotor research has concentrated on improving hovering efficiency, requiring only those pitching schedules achievable with a simple mechanical linkage. No pitch control apparatus has been developed to actuate and control the dynamic blade kinematics necessary for efficient forward flight.
Similar to understanding the meaning of cyclic and collective pitch control as used for helicopters helps understanding the importance of the same terms as used for cyclorotors, understanding the meaning, and history, of the term cycloidal will help understanding of cycloidal rotors.
As described, for example, in U.S. Pat. No. 2,580,428 to Heuver, the rotors on a cyclogyro are called cycloidal rotors, or propellers, because the path followed by the long axis of each rotor blade approximates a cycloid. A cycloid, as shown in FIG. 2, is the path 20 followed by a point 22 on a rotating circle 24. Because the blades are rotating about the rotor axis while the aircraft is also moving forward, their long axes will follow a pure cycloidal curve only if the rotor axis of rotation advances a distance during a single rotation equal to the circumference of the rotor, or Pi times its diameter. This translation advance per revolution of the rotor is conventionally called pitch.
It is important to recognize that the conventional use of the term pitch, how far an aircraft propeller or a ship's screw moves forward during one complete revolution, while useful to traditional propeller designers, is not used for cyclorotors.
The technical definition of pitch for cyclorotors refers to the angle the chord of each individual cyclorotor blade makes with a line tangent to the blade's path around a cyclorotor's axis of rotation.
For forward motion to occur, each blade must follow a so-called prolate cycloid path, where a point on a circle, or the long axis of a rotor blade about the rotor axis, will follow a path greater than Pi times the rotor diameter. An example of a prolate cycloid path 30 is shown in FIG. 3, where a circle 32 representing a rotor is mounted concentrically on a larger circle 34 which rolls along a plane surface 36.
FIG. 4 is an example of a so-called curate cycloid path 40, where a point 42 is on a circle representing the circumference of a rotor and where the circle has a greater diameter than a circle 44 rolling along a plane surface 46.
FIGS. 2, 3 and 4 are modified from images available at the Wolfram MathWorld web site, and are used with sincere thanks. The descriptions of which path is a curate cycloid path and which a prolate cycloid path at the Wolfram MathWorld web site, the arbiter for standard math usage, may, in fact, more accurately be reversed when applied to cyclorotors, and are presented here only for background.
It's important to note that curtate and prolate advance ratios describe a desired overall result, and not how to achieve that result. How to achieve that result is the problem the prior art has sought to solve.
A three-bladed cyclorotor 50 is shown in FIG. 5. As described earlier, cyclorotors are characterized by the rotation of blades 52 about a horizontal axis where the span 54 of the blades is parallel to the axis of revolution 56 and perpendicular to the direction of flight 58. Also as described earlier, forces are generated by cyclically pitching the blades as they move around the rotational axis so that, for example, in a hover, a positive pitch on the top half of the cycle and a negative pitch on the bottom half produces an upward force. By varying amplitude and phasing of cyclic pitching magnitude, the direction of thrust can be varied.
The cyclogyro flown at the University of Maryland in 2011 used the blade pitching mechanism shown in FIG. 6. A link 62 connects an offset eccentric ring 64 to the aft end of each blade 66. Eccentric ring 64 is offset from the rotational axis of the cyclorotor at a fixed distance, but can be rotated by a servo to change the phase of the blade pitching and thereby the direction of thrust. In effect, these rotors have cyclic, but not collective, pitch control. The magnitude of thrust is varied only through the rotational speed of the rotors.
A detailed description of the operating principals of cycloidal rotors, including a discussion of prior art attempted solutions, is found in Investigation and Characterization of a Cycloidal Rotor for Application to a Micro-Air Vehicle, Eric Parsons, M.S. Thesis, University of Maryland (2005), which is incorporated by reference into this description.
Even though the University of Maryland control strategy is simple and reliable, it is inherently limited. Most significantly, it cannot provide the range of blade kinematics required for efficient forward flight, which must include both cyclic and collective pitch control. Additionally, as forward velocity increases, the pitching kinematics of the blades must change significantly to account for an additional horizontal component of flow. The existing blade pitch control strategy shown in FIG. 6, even if collective pitch control were added, does not permit such pitching schedules.
The need for pitching schedule complexity results, at least in part, from the relative direction of flow on each blade. Similar to conventional rotors, this is determined by velocity due to rotor rotation, the free stream velocity, and velocity due to inflow. The advance ratio μ, as shown in equation (1), is the ratio of free stream velocity to the velocity due to rotation, and is the driving representation of these velocity components. The forces on each blade are dependent on the angle of attack of the blade which, in turn, is determined by the relative direction of flow. Thus, the desired pitching schedule is a function of this ratio.
                    μ        =                              V            ∞                                Ω            ⁢                                                  ⁢            r                                              (        1        )            
FIGS. 7a-7b show snapshots of an airfoil 72 aligned with the relative flow as it travels counterclockwise around the rotational axis 74 of a cyclorotor 76 at advance ratios of zero (a), one (b) and two (c). Advance ratios less than one are referred to as curtate advance ratios while those greater than one are referred to as prolate advance ratios. An advance ratio of one, where the velocity from rotation is equal to the free stream velocity, is a cycloid. Advance ratios near one are unusual as a 180 degree blade pitching change is necessary (near the bottom of the rotation) as the blade retreats and the flow direction relative to the blade changes. The flight criterion to operate at prolate advance ratios demands that the pitch schedule of a blade needs to monotonically increase throughout the cycle of a revolution from φ=0, 2π, 4π, . . . m/π, where m is the number of revolutions. This blade motion simply aligns the blade chord with the flow, so that to produce force the blades must be pitched cyclically relative to these zero angle of attack positions.
As previously described in relation to FIG. 6, traditional control mechanisms pitch the blade by attaching control rods or links from the blade to a rotating eccentric ring. By varying the position of the eccentric ring, the blades are pitched in an approximately sinusoidal manner with variable amplitude and phase. This sinusoidal pitching schedule, however, cannot efficiently compensate for increasing free stream velocities because the fraction of rotation at which the pitch of the blades can be optimized decreases with increasing advance ratio as the optimum pitching schedule becomes non-sinusoidal. Thus, these mechanisms are limited to operation at low curtate advance ratios.
Pitching schedules that more accurately account for incoming flow direction will be more efficient at all advance ratios because the pitch of the blades can always be optimized. However, no mechanism has been developed to produce all necessary pitching schedules. The Parsons M.S. thesis includes a suggestion to use multiple cams or to control each blade individually. However, multiple cam mechanisms are extremely complicated and are only efficient at specific advance ratios, while individual blade control by electronics or hydraulics is impractical at high rotational speeds because the necessary large blade rotational accelerations cannot be achieved with the speed and magnitude required.
There is, therefore, a need for new apparatus and methods that can dynamically control the pitch of cyclorotor blades, particularly for producing diverse pitching schedules.
Such new apparatus and methods should be able to provide not only cyclic and collective pitch control, but control blade pitch to optimize for the direction of flow over the blades.
Such new apparatus and methods must also be able to provide actuation forces sufficient to achieve the necessary blade rotational accelerations.
A solution for the problems of dynamic control of cyclorotor blades will reveal similar needs for new apparatus and methods for dynamically controlling other moving apparatus parts.